There have been for many years systems to convert cinematographic motion picture film into electrical signals for television broadcast or video cassette production. These machines are commonly referred to as ‘Telecine’ machines. Such examples of these machines includes the ‘URSA’ telecine from Cintel International Ltd, and the ‘SPIRIT’ Telecine from Phillips Digital Video in Darmstadt, Germany, and the applicant's own ‘Millenium Machine’ These telecine machines need to ‘scan’ the cinematographic film. This is accomplished in the SPIRIT system by the use of a CCD (Charge Coupled Device) and more relevantly to this invention, by the use of a CRT (Cathode Ray Tube) in the Cintel International URSA machine.
The use of CRT (Cathode Ray Tubes) for the use of film scanning is not new. It has been known at least as long ago as 1975, where it was used in the Rank Cintel Mk III Telecine. In this system, the film is ‘scanned’, by the imaging of a scanned ‘raster’ patch on the front of the CRT being imaged onto the film, and the collected light transmitted through the film being collected by photosensitive devices. These photosensitive devices are quite often photomultiplier tubes. These photomultiplier tubes convert the incident light into an electrical signal that is dependant on the amount of light, and thus dependant on the density of the film at any point scanned. It is well known that such devices require a voltage to be applied to them, and that the signal resulting from these devices is dependant on the voltage level applied to the device.
It is often necessary to calibrate this light measuring system, for many reasons. Firstly it is desirable to have a measurement of the actual density of the film present in the telecine gate, rather than just a relative measurement of how much lighter or darker one portion of the film is with respect to another. Secondly, the light emitted from the CRT is variable, and as a CRT ages, the light emitted from the phosphors tends to decrease for a given drive current. Further, the signal received from the photomultipliers is dependent on the electrical gain provided on these devices. There is also potentially an ‘aging’ effect in the photomultiplier tubes, resulting in less signal for a given amount of incident light and gain voltage.
Telecine machines are often used in conjunction with Telecine programmers. Such devices include the ‘POGLE Platinum’ from Pandora International Ltd., of Northfleet, Kent, England. These devices are used to ‘store’ control adjustment parameters for later recall. These parameters relate to editorial alterations to particular scenes (or even film frames) of a given film reel. Naturally, it is desirable to be able to view the ‘red’ colour of a woman's dress from a particular scene on a roll of film, and several weeks later to view that same scene, with the same stored control parameters, and see the same ‘red’ hue. Thus the stored telecine parameters must produce the same rendered colours day-in and day-out. The electrical signal necessary to be applied to the photomultipliers to produce that hue from that film will vary slightly, according to the age of the photomultiplier devices, thermal drift in the control circuitry, and thermal effects in the photomultiplier. It can be seen that some ‘calibration’ mechanism is necessary to be able to reproduce hues repeatably and reliably.
There have been attempts to regulate the CRT system. For example, it has been known for many years to use a technique called ‘burn correction’. This basic technique was disclosed in the now abandoned German patent application DE-OS-25 25 073, published on 18th Dec. 1975. Burn correction works by measurement of the emitted light from the CRT at any moment in time both directly from the CRT face, to derive a ‘burn’ signal, and also through the film, to derive an image signal. Thus if the part of the CRT face we are using (or indeed the whole CRT face) is emitting less light than elsewhere (or previously) then we can correct for this in the light collected after passing through the film. For example, if the CRT light were to drop by 10% as measured by the ‘burn’ corrector, the light collected by the image sensing photomultipliers would be measured relative to this ‘90%’ light value rather than the assumed 100% that would be assumed without ‘burn’ correction. Having effectively provided a control mechanism to ensure that the CRT at least appears to give a constant amount of light, the area that we wish to describe here is the calibration of the light sensors.
There have been several proposals in the attempt to calibrate the photomultiplier response to the light level from the CRT. One such proposal is disclosed in patent GB 2 241 625, by Rank Cintel Ltd. This method teaches how to carry out a ‘two point calibration’ for the system. The steps for this are to set the CRT beam current to a nominal ‘low’ level, typically 15 Microamps, and to then alter the PEC drive voltage to produce a ‘peak electrical signal’. This is repeated with a ‘maximum’ beam current, typically 300 microamps, where the PEC drive voltage is again varied to get the ‘peak electrical signal’. This data is used to create a ‘straight line relationship’ between PEC gain and PEC drive voltage.
This method is inherently flawed, as the characteristic of PEC gain to drive voltage departs significantly from a linear or exponential characteristic at certain drive levels and is not repeatable from one PEC to another.